Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to improved means for attaching a tensed foil color selection electrode to an electrode support structure. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the foil-type flat tension color selection electrode and flat faceplate provides many benefits in comparison to the conventional domed color selection electrode and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved color selection electrode, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities become visible as the electrode moves closer to the faceplate and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate. The tension foil color selection electrode, when heated, distorts in a manner quite different from the conventional color selection electrode. If the entire electrode is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the electrode are heated, those portions expand, and the unheated portions contract, resulting in displacements within the plane of the electrode; i.e., the electrode remains flat.
The tension foil color selection electrode is a part of the cathode ray tube faceplate assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen which consists of deposits of light-emitting phosphor elements, a color selection electrode, and support means for the electrode. As used herein, the term "color selection electrode" means an apertured metallic foil which may, by way of example, be about one mil thick, or less. The electrode must be supported under tension a predetermined distance from the inner face of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the color selection electrode, also called a "parallax barrier," ensures that each of the three beams of the color tube lands only on its assigned phosphor elements.
The requirements for a support means for tension foil color selection electrodes are stringent. As has been noted, the foil color selection electrode is normally mounted under high tension. The support means should be of high strength so that the electrode is held immovable; an inward movement of the electrode of as little as one-tenth of a mil is significant in expending guard band. Also, it is desirable that the color selection electrode support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner face of the faceplate, the support means should have a coefficient of thermal expansion compatible with the glass, and by its composition, be bondable to glass. Also, the support means should be of such composition and structure that the color selection electrode can be secured to it by production-worthy techniques. Further, it is essential that the support means provide a suitable surface for mounting and securing the electrode. The material of which the surface is composed should be adaptable to machining or other forms of shaping so that it can be contoured into near-perfect flatness so that no voids between the metal of the electrode and the support structure can exist to prevent the positive, all-over contact required for proper electrode securement.
A present means for installing a color selection electrode is to stretch or otherwise expand an in-process color selection electrode across a metal faceplate-mounted frame by suitable tensioning means. The electrode is expanded across the frame and can be secured to the frame by processes such as laser welding, and electrical resistance, or "spot" welding. In a 14-inch tube, for example, more than 1000 such welds at intervals of about 0.040 inch are required around the circumference of the frame to ensure positive securement of the electrode. A disadvantage of spot welding by laser beam welding and electrical resistance is that the securement of the electrode is at intermittent locations, rather than along a continuous, uninterrupted line. Laser spot welding is also sensitive to an irregular contour of the electrode mounting surface, which can produce unweldable gaps between the electrode and the mounting surface. Variance in electrode thickness can also be a detriment to laser spot welding. Another factor adversely affecting both welding means is vulnerability to the presence of contaminants such as PVA, grille dag, and oxides, which can affect the integrity of the weldments. A further disadvantage in both types of welding is the intensity of the heat that must be applied at the weld; e.g., 2,800 degrees F.--a temperature level which can anneal and weaken the hardened metal of the electrode not only at the point of the weld, but over an appreciable area in the vicinity of the weld. The effect can be an appreciable loss of tension of the electrode originating at the points of attachment in peripheral areas of the electrode. As a result, color purity of the image can be affected.
A continuous roller or "seam" weld accomplished by electrical resistance can provide a continuous weld; however, as with laser and spot welding, the intensity of the heat applied at the weld line would tend to anneal and thus weaken the electrode in the peripheral areas, leading to loss of tension.
In U.S. Pat. No. 2,842,696 to Fischer-Colbrie, a color cathode ray tube is disclosed in which a color selection electrode consists of a grid of wires stretched in parallelism with a flat faceplate. In one embodiment of the invention, the ends of the wires are held in place and taut by means of an electrically conductive cement consisting of a mixture of powdered aluminum silicate and aluminum phosphate in a paste-forming medium of a dilute water solution of phosphoric acid. Also described is a means of bonding in which the ledges that receive the wires of a wire-type electrode are (to quote) " . . . suitably metallized in a well-known manner in which case the bonds can be `metallic or metal-to-metal`." No particulars as to this means of bonding are disclosed.
Bonding of metals is described in a book titled Capillary Joining--Brazing and Soft-Soldering, by C. J. Thwaites. (Research Studies Press, John Wiley & Sons, Ltd. 1982.)